Effects of glucose deprivation and altered Ca²⁺ concentrations on clonal insulin-producing cells (RINm5F)

The effects of various Ca²⁺ concentrations and glucose deprivation on cell proliferation, insulin secretion and contents of insulin, calcium and magnesium were studied in a clonal cell line (RINm5F) established from a transplantable rat islet tumor. The RINm5F cells were equally effective in proliferating and releasing insulin at Ca²⁺ concentrations ranging from 0.16 to 4.4 mM. In contradistinction to normal β-cells, the amounts of calcium in the RINm5F cells remained relatively constant after lowering the extracellular concentration of Ca²⁺ and inhibiting the entrance of the ion with D-600. However, culture at 4.4 mM Ca²⁺ resulted in substantial incorporation of cellular calcium and a simultaneous reduction of the insulin content. Cells deprived of glucose had a significantly lower dry weight, were less able to exclude trypan blue and contained and released less insulin. Since protein biosynthesis is strictly dependent upon an adequate intracellular concentration of Mg²⁺, these long term effects of glucose removal might be related to the concomitant decrease in the cellular content of magnesium.